fix(ui): a late form takes a dialog's focus once, even if it arrived elsewhere

The watch stayed armed when the form arrived while the focus was off the close button, so a
later change in the body pulled the focus off a close button the user went back to. It now
disarms on arrival and re-arms only when the focus is taken away.

frontend/src/components/ui/Modal.tsx

@@ -106,9 +106,12 @@ export default function Modal({
   // A form that loads after the dialog opens (the issue composer shows a spinner first) has no field
   // to take the focus at open, so it lands on the ✕. When the first control arrives the focus moves
   // there — but only while it still rests on the ✕: a user who has moved it keeps it where it is.
-  // The watch re-arms when the focused control is taken away (a failed load's Retry giving way to
-  // the spinner again): the focus fell to <body>, so it waits on the ✕ for the form (E32 review).
-  // Only <body>: focus in a dialog stacked on top is not this one's to take.
+  // The form takes it ONCE: arrived while the focus was elsewhere, it disarms all the same, or a later
+  // mutation would pull the focus off a ✕ the user went back to (E32 review round 2). The watch
+  // re-arms only when the focus is taken away, not moved: the body has no control again, or the
+  // focused one went (a failed load's Retry giving way to the spinner, or straight to the form) and
+  // the focus fell to <body> — it then waits on the ✕ for the form (E32 review). Only <body>: focus
+  // in a dialog stacked on top is not this one's to take.
   useEffect(() => {
     const body = bodyRef.current;
     if (initialFocus !== "field" || !body || focusablesIn(body).length > 0) return;
@@ -119,9 +122,13 @@ export default function Modal({
         armed = true;
       }
       const first = focusablesIn(body)[0];
-      if (!armed || !first || document.activeElement !== closeRef.current) return;
+      if (!first) {
+        armed = true;
+        return;
+      }
+      if (!armed) return;
       armed = false;
-      first.focus();
+      if (document.activeElement === closeRef.current) first.focus();
     });
     watch.observe(body, { childList: true, subtree: true });
     return () => watch.disconnect();